Nephrotic syndrome (NS) is one of the most common kidney diseases seen in children. It is characterized by massive protein leakage across the kidney's filtration barrier and dramatic structural and cytoskeletal changes in podocytes, which in part comprise this barrier. Glucocorticoids are the primary therapy used to treat NS, though neither the target cell nor their mechanism of action in NS are known. Due to the absence of inflammatory cells in the glomerulus during idiopathic NS and the known immunosuppressive properties of glucocorticoids, soluble mediators released from circulating lymphocytes have historically been presumed to be responsible for disease, and the clinical efficacy of glucocorticoids assumed to result from effects on lymphocytes. In contrast, we recently identified a novel direct protective effect of glucocorticoids on podocytes in culture. We found that glucocorticoids can both protect and enhance the recovery of cultured podocytes from both injury induced by the podocyte toxin puromycin aminonucleoside and from direct disruption of the actin cytoskeleton by latrunculin A. Protection is abolished by RU486, which blocks glucocorticoid receptor-mediated transcriptional activation, suggesting that these effects are mediated by changes in protein expression. Our proteomic analysis of protein expression in podocytes treated with glucocorticoids identified and confirmed induction of alphaB-crystallin (aBC) and hsp25, and we subsequently showed that altered expression of these proteins can modulate the protective effects of glucocorticoids in cultured podocytes. Based on these novel observations, we hypothesize that the beneficial clinical effects of glucocorticoids in NS are mediated, at least in part, by a direct action on podocytes to induce expression of aBC and/or hsp25. To test this hypothesis, we will measure: 1) the effects of an inducible, podocyte-specific knock-in of a highly-active mutant glucocorticoid receptor on the therapeutic action of glucocorticoids in experimental NS, and changes in the expression, localization, and phosphorylation of aBC and hsp25 in response to glucocorticoids and podocyte injury, 2) the effects of alterations in the expression, phosphorylation, or known mechanisms of action of aBC and/or hsp25 on podocyte injury and the response to glucocorticoids. Relevance: Identification of a podocyte mechanism of action of glucocorticoids in NS would permit the development of more targeted and less toxic therapies.
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|Chittiprol, Seetharamaiah; Chen, Phylip; Petrovic-Djergovic, Danica et al. (2011) Marker expression, behaviors, and responses vary in different lines of conditionally immortalized cultured podocytes. Am J Physiol Renal Physiol 301:F660-71|
|Guess, Adam; Agrawal, Shipra; Wei, Chang-Ching et al. (2010) Dose- and time-dependent glucocorticoid receptor signaling in podocytes. Am J Physiol Renal Physiol 299:F845-53|